<html>
<head><title>Calculator</title></head>
<body>

<h1>Calculator</h1>
Type an expression in the following box (e.g., 10.5+20*3/25).
<p>
<form method="GET">
<input type="text" name="expr"><input type="submit" value="Calculate">
</form>
</p>
<ul>
<li>Only numbers and +,-,* and / operators are allowed in the expression.
<li>The evaluation follows the standard operator precedence.
<li>The calculator does not support parentheses.
<li>The calculator handles invalid input "gracefully". It does not output PHP error messages.
</ul>
Here are some(but not limit to) reasonable test cases:
<ol>
  <li> A basic arithmetic operation:  3+4*5=23 </li>
  <li> An expression with floating point or negative sign : -3.2+2*4-1/3 = 4.46666666667, 3+-2.1*2 = -1.2 </li>
  <li> Some typos inside operation (e.g. alphabetic letter): Invalid input expression 2d4+1 </li>
</ol>
<?php
	$exp = $_GET["expr"];
	preg_match("/^[0-9+-\/\*]*/i", $exp,   $matches);  	
	preg_match("/\/0/i",$exp,$zero);
	//preg_match("/[+-\*\/]{2,}/i",$exp,$minmin);
	if($exp!="" && $matches[0]==$exp && $zero[0]=="")// && $minmin[0]=="")
	{
		eval("\$ans = $exp ;");
		echo "<h2>Result</h2>";
		echo $exp." = ".$ans;
	}
	else if($exp !="")
		echo "Invalid Expression";
?>
</body>
</html>

